This invention relates to a thermal link which is placed between a source of heat and a heat sink when the source of heat cannot be cooled directly. One common example of the use of a thermal link is to carry the heat produced by a semiconductor device or a circuit board containing many semiconductor devices to an external heat sink when cooling these devices directly by placing them in a fluid flow is undesirable or impossible.
As is well known in the art, air is a poor conductor of heat. Therefore, it is necessary that the thermal link make a good contact with both the heat source and heat sink so as to exclude or minimize the amount of air trapped therebetween. The use of fillers, such as thermally conductive grease, between the heat source or heat sink and the thermal link is undesirable because such materials tend to attract and hold dirt which may then interfere with the efficient transfer of heat. In addition, these greases are subject to drying up with age or running at high temperatures, which creates maintenance problems and limits its useful life as an efficient heat transfer system. Permanently attaching the thermal link to both the heat source and the heat sink would overcome these problems but is undesirable because it makes the apparatus very difficult to maintain.
These problems are further exacerbated when the heat source and/or the heat sink do not have precision machined flat surfaces so that air pockets can be created at the contact point with the thermal link. The problem can be further complicated if the heat source and the heat sink are so mounted that they are not uniformly spaced from each other, as might be the case, for example, when the mounting configuration is not made from precision made parts.
U.S. Pat. No. 3,950,057 shows a printed circuit card guide which can be utilized to transfer heat from the printed circuit card to an external heat sink. In one embodiment, the printed circuit card is held by a series of arcuate spacers and cantilevered spring fingers which are mounted in a U-channel which in turn is fastened to a frame. In another embodiment the printed circuit card is engaged by means of a strip of flexible metal formed to include opposed, cantilevered flexible spring fingers which in turn are mounted within a U-shaped member. This design does not provide for efficient heat transfer because the heat is transfered from the edges of the printed circuit card, and the printed circuit card engaging means within the guide provide a very small contact area with the card.
It is well known in the art of electromagnetic interference (EMI) shielding to use a strip of finger stock material to bridge the gap between two surfaces. Finger stock material comprises a U-shaped ribbon of spring material having cantilevered spring fingers stamped into one side. The ends of the cantilevered fingers contain arcuate-shaped contacts. The finger stock material is mounted by attaching the side not containing the fingers to a first surface so that the side makes electrical contact with the surface. A second surface touches the surface of the arcuate contacts to make a point-contact electrical connection, thus providing EMI shielding.
The point-contact made by the finger stock does not provide for an efficient thermal transfer between the two surfaces. Flattening the arcuate contacts will not solve this problem because the resulting structure will transmit the forces applied to the fingers to the joining member formed at the base of the U-shaped ribbon, causing it to permanently deform. Once this joining member is permanently deformed, good contact cannot be maintained between the fingers and the surface mating with the fingers, resulting in poor heat transfer.
One solution which readily presents itself is to force together two surfaces which are not in adequate thermal contact by simply using more pressure. In fact, the United States Navy has proposed in the Thermal Application Handbook for the Standard Electronic Modules Program TP 529, dated March 1981 to build a heat sink assembly for a printed circuit card cage in which powerful springs are utilized to force the top fin of the aluminum carrier for the printed cirucit card into contact with an external heat sink. In the particular embodiment disclosed, a net force of 100 pounds per module was utilized to force the surfaces of the printed circuit card carrier and the heat sink into intimate contact. In view of the fact that a large number of modules are normally utilized in such card cages, a large net force is placed upon the heat sink. For example, a card cage containing only 10 modules would have a net force of 1,000 pounds applied to it. This requires a heat sink which is exceptionally rigid, thereby increasing the weight, bulk, and cost of the assembly. In addition, a mechanism is required to apply and release the pressures generated by the springs so that the assembly can be disassembled for maintenance or replacement of individual circuit cards.
It is also known to use thermally conductive particle filled elastomeric materials as thermal links. These devices typically take the form of electrical insulators for semiconductor devices which replace the conventional mica insulator covered with heat conducting grease. These insulators take the form of a thin film which is cut to fit the mounting of the semiconductor device and held in place by the mounting hardware for the semiconductor device. These materials have a high durometer and thus require high pressure in order to conform to irregularities in either the heat source or heat sink or to compensate for nonuniform spacing in the mounting of the heat source and the heat sink relative to each other. This does not pose a problem where these materials are utilized to mount, for example, a single transistor by means of screws. However, if a large number of devices or modules are linked to a single heat sink, the force applied to the heat sink can be excessive, creating the weight, bulk and cost problems described above.